An undesirable feature and often difficult problem to solve when designing a narrow band SAW filter, especially on quartz, is presented by a presence of transverse modes. With a finite aperture for a SAW device, several guided modes can propagate instead of one. The effect of these transverse modes is to add several other undesirable responses to an otherwise ideal filter response, shifted in frequency and to a lower level. The result can produce both ripple in the pass band and spurious rejection. Several approaches have been used to solve this problem.
One approach reduces the transducer aperture, which reduces the number of guided modes and thereby increases their frequency difference. Typically, the aperture will be chosen in order to have only one guided mode. The drawback of this approach is that a small aperture has to be chosen, resulting in high impedances.
Another approach is to apodize the transducer in order to get more coupling to the main mode and to reduce the coupling to the other modes. While this method results in some improvement, it is limited by the main transverse mode extending outside of the acoustic aperture, making it difficult to construct an apodization desirably reproducing the mode shape.
Yet another approach was described in a publication to M. Mayer et al. for Low Loss Recursive SAW Filter for Base Station Applications without Spurious Modes' (2005 IEEE Ultrasonics Symposium, proceedings pp 1061-1064) and in US Patent Application Publication No. US 2007/0018755 A1 to Mayer et al. for Acoustic Wave Transducer with Transverse Mode Suppression, the disclosures of which are herein incorporated by reference in their entirety. A proposed alternate solution is similar to the piston mode approach in BAWs is illustrated in publication for “Design Modeling ands Visualization for Low Transverse Modes R-SPUDT Devices. As illustrated with reference to FIG. 1, the principle is to add a lower wave velocity region to the edge of the transducer so as to produce a mode constant in the aperture and decreasing outside, as further illustrated with reference to FIG. 1A. The shape of this mode is close to the rectangular shape of the source so that only this mode should be excited. A region with smaller electrode period is used so that the velocity is decreased as a result of the topographic effect. A velocity shift is proportional to the number of electrodes per wavelength. Such an approach may be used in a single phase unidirectional transducer (SPUDT), by way of example. Some improvements in the filter response result, but parasitics still remain. The present invention seeks to further improve an implementation of such a piston mode device.